A voltage-controlled oscillator (VCO) refers to an oscillating circuit for which a correspondence relationship between an output frequency and an input control voltage exists.
The voltage-controlled oscillator is one of important basic circuits in the integrated circuit. The implementation of the voltage-controlled oscillator includes a ring voltage-controlled oscillator (Ring VCO) and an inductance-capacitance voltage-controlled oscillator (LC VCO). The voltage-controlled oscillator is widely applied to a clock synchronization circuit in a microprocessor, a frequency synthesizer in a wireless communication transceiver, a multi-phase sampling circuit and a clock recovery circuit (CRC) in optical-fiber communication.
Phase noise is one of main parameters for measuring performance of the voltage-controlled oscillator. In most cases, the phase noise performance of the voltage-controlled oscillator is the dominating factor for sensitivity of an integrated receiver. Ideally, a signal spectrum output by a voltage-controlled oscillator is an impulse function. However, signal spectrum characteristics output by the voltage-controlled oscillator is a frequency response masking curve since various noise sources exist in practical circuit.
The noise source in the voltage-controlled oscillator circuit may include a device noise and an external interference noise. The device noise mainly includes a thermal noise and a flicker noise, and the external interference noise mainly includes a substrate noise and a power supply noise. The device noise in the voltage-controlled oscillator mainly comes from a parasitic series resistance of an on-chip inductor and a variable capacitor, a switch differential pair tube and a tail current source.